Nearly 20 million Americans report use of cannabis in the past month, and heavy cannabis use has increased by nearly 60% in the U.S. since 2007. Heavy cannabis use is associated with lower educational attainment, reduced physical activity, and increased rates of addiction, unemployment, and neuropsychological deficits. Studies by our lab and others suggest that cannabis use is also associated with increased mental health symptoms and suicidal and nonsuicidal self-injury. In addition, cannabis is the illicit drug most strongly associated with drugged driving and traffic accidents, including fatal accidents. While there is evidence that sustained abstinence can lead to improvements in the functional outcomes of former users, the degree to which reductions in cannabis use alone (i.e., in the absence of sustained abstinence) might be associated with positive changes in functional outcomes is currently unknown. This is a critical gap in the literature, as many clinical interventions for cannabis and other drugs are associated with decreases in frequency and quantity of use, but fail to achieve an effect on overall abstinence rates. The overall objective of the present research is to use ecological momentary assessment (EMA), a real-time, naturalistic data collection method, to prospectively study the impact of reduced cannabis use on functional outcomes in heavy cannabis users. EMA addresses several limitations of traditional assessment techniques by enhancing ecological validity, minimizing memory bias, and enabling examination of the impact of context on participants' behavior. Contingency management (CM) will be used to promote reductions in frequency and quantity of cannabis use. CM is an intensive behavioral therapy that is highly effective at producing short-term reductions in illicit drug use. Moreover, we have recently developed a novel approach that leverages mobile technology and recent developments in cannabis testing to make CM for cannabis more portable and feasible. We have pilot-tested this approach with heavy cannabis users and found that it is an acceptable and feasible method to reduce their cannabis use. The present research will use this technology in conjunction with state-of-the-art EMA methods to study the impact of reduced cannabis use on key functional outcomes. Our central hypothesis is that reductions in frequency and quantity of cannabis use will lead to positive changes in cannabis users' mental health, physical activity, working memory, health-related quality of life, and driving behavior. The rationale for this research is that it will provide the first and only real-time data concerning the potential impact of reductions in cannabis use on functional outcomes. As such, the findings from the present research will directly inform ongoing efforts to include reductions in illicit drug use as a valid, clinically-meaningful outcome measure in clinical trials of pharmacotherapies for the treatment of substance use disorders.